1. Field of the Invention
The present invention relates to a tapered end mill for machining or cutting rib slots or tapered slots in metal molds for casting rib section in resin products or die cast metal products.
2. Prior Art
It is well known that electrical discharge machining or cutting is generally used to cut tapered slots (rib slots) for casting rib section in resin products or die casting metal products.
Electrical discharge machining uses an electrode having a shape corresponding to a rib slot of a metal mold and thus requires many processes: electrode forming, electrical discharging and polishing processes. These processes also require corresponding processing machines. Despite these problems, electrical discharge machining is frequently used for cutting deep rib slots (with a high ratio of depth to width) or when rib slots cannot be cut using a cutting tool due to limitations in the slot shape.
When cutting a rib slot using a cutting tool, as shown in FIG. 9, a generally-used square end mill cuts a rectangular cross-sectional slot section 2 which extends from the top 6 to the bottom 7 of the slot by repeating a cutting stroke in the longitudinal direction of the slot. Then a tapered end mill B having helix edges b.sub.2 at its tapered section b.sub.1 as shown in FIG. 8 is used to remove tapered sections 3. In the cross-sectional shape of a rib slot, the depth 5 of the slot is far greater than (five times or more) the width 4 of the slot. If the entire rib slot is cut by one process, the end mill must be extended further and the amount of cutting chips increases. The stiffness of the end mill reduces and cutting is impossible. Therefore, even when a single machine can be used for cutting unlike electrical discharge machining, costs for cutting tools are high and cutting efficiency is low.
A tapered end mill A with a polygonal cross-section shown in FIG. 7 is used to efficiently cut rib slots. More particularly, the tapered end mill(a) has a regular polygonal shape in the cross section perpendicular to the axis of the end mill at the tapered section a.sub.1. Each edge a.sub.2 is formed at the corresponding vertex of the regular polygonal shape along the axis and has a helix form. The edge is also formed along the imaginary tapered cylindrical surface which circumscribes the regular polygon. In addition, end cutting edges a.sub.3 and peripheral cutting edges a.sub.4 are formed at the smaller end of the tapered section a.sub.1. With this tapered end mill (a), the cross-sectional area of the regular polygonal tapered section a.sub.1 connected to the edge section at the smaller end is larger than that of an ordinary end mill. Therefore, the end mill is superior in stiffness and is less deformed by cutting force. Furthermore, each edge of the regular polygon functions as a wiper and provides a burnishing effect. These features eliminate the need for the preliminary process of cutting the above-mentioned rectangular section. With this single end mill, both the cutting and burnishing processes can be done simultaneously and quickly. When cutting a deep slot (such as a rib slot with a high ratio of depth to width), the tool requires large stiffness and superior cutting performance. Countermeasures should also be taken to reduce deflection during cutting and to maintain high cutting accuracy. An ordinary slot end mill has right hand helix edges to provide superior cutting performance and to discharge chips upward from the end cutting edge. In the case of the above-mentioned end mill with a polygonal cross section, its helix angle becomes smaller due to cutting force during cutting and self-excited vibration occurs sometimes in the direction of cutting into the bottom of the slot. This results in unstable cutting performance and breakage of the end mill.
To solve these problems, the tapered section of an end mill provided with a regular polygonal cross section and right hand helix edges is found in Japanese Utility Model Publication No. 63-161615 (applied by the same assignor as that of this patent application). Since this end mill has a polygonal cross section, its stiffness, to which cutting performance is dependent on, is greater than that of a conventional end mill equipped with two cutting edges.
However, since the end mill of the prior application has right hand helix edges and starts cutting at the smaller end of the cutting edge, its cutting performance is not stable. The finished surface is rough and the cutting depth is smaller as clearly shown in Table 1 and FIG. 10 (comparison).